1. Field of Invention
This invention relates to thermoplastic anti-corrosion coatings, particularly primer coatings for metals. More particularly, it relates to copolymers of ethylene with a co-monomer containing a glycidyl group, optionally with a filler such as zinc, which can be applied as a thermoplastic coating to prevent corrosion of metals. The coating may be used alone as a sole coating but preferably is used as a primer coating under an outer coating that provides mechanical protection for the primer coating.
2. Background Discussion and Related Art
Metal vessels, pipes and other forms used for containing and transporting a variety of materials are subject to corrosion or erosion by the contained or transported materials. Metal objects are also subject to corrosion or erosion by the environment with which they come into contact. For example, soil, salt water or atmospheric and climatic conditions can have a harsh effect on metal.
To protect against such corrosion and erosion, metals are commonly coated with plastic materials. In addition to providing protection against corrosion or erosion, certain plastic coatings provide desirable properties inherent in the plastic being used. For example, a very smooth surface can reduce the coefficient of friction in a pipe thus reducing the energy needed to pump a fluid through the pipe.
To provide effective protection against corrosion, a coating should have good adhesion to the metal and should be relatively impermeable to agents which could, in themselves, cause corrosion of the metal or to agents which cause a loss of adhesion of the coating to the metal. Poor initial adhesion or subsequent loss of adhesion will allow the metal itself to become directly exposed to corrosive environments. Thus, both impermeability and long term adhesion are important characteristics of a good corrosion-prevention coating.
Coating materials differ in their advantages, however. Polyolefin thermoplastic coatings such as polyethylene or polypropylene are resistant to water and chemicals, but they do not adhere well to metals. By contrast, ethylene copolymer ionomers, which are ion-neutralized ethylene (meth)acrylic acid copolymers, provide a high level of adhesion to metals, are tough and provide good abrasion resistance, but are water sensitive and permeable.
Adhesion and permanence of that adhesion to metals are complex phenomena. Loss of adhesiveness may be due to mechanical or chemical causes. Differential thermal expansion of the metal and the coating can cause mechanical failure of the bond between them, while many agents can attack the metal-coating bond.
Since all of the qualities of a good coating (relative impermeability to potentially corrosive agents plus good and lasting adherence under a wide range of conditions) are not always possible in one coating, it is common to use primer coatings between the metal and an outer plastic coating to provide permanent adhesion between the metal and outer coating, yet maintain the advantages of the outer coating.
Thermoset epoxy primers or sole coatings are known and among the preferred materials for primers or sole coatings. Thermosets have the advantage of relatively low coefficient of expansion and less differential coefficient of expansion with metals. They are, however, quite brittle and are therefore used in quite thin layers. Moreover, they must be cured. Thermoset epoxy resins are excellent adhesives but do not necessarily provide ideal coatings for many purposes.
Particulate fillers, particularly those laminar in shape, are commonly used in coatings to improve corrosion resistance. They aid in reducing differential coefficient of expansion and may reduce permeability by increasing tortuosity of the path that would be required for a fluid to permeate the coating. Particulate zinc as a filler is particularly advantageous because it has yet another corrosion protective function, which is related to its reduction potential. It is known for use in coatings and paints. Use of zinc itself as a protective coating is of course long known, particularly with steel because of its reduction or galvanizing potential.
U.S. Pat. No. 3,201,497 (Heino) describes a thermoset epoxy coating adhesive which can be used as a protective coating, as well as for bonding metals to each other. Metals include steel, aluminum, copper and chromium. The epoxy resin may contain a co-monomer such as glycidyl methacrylate which acts as a `reactive flexibilizer` during the curing of the resin. A variety of fillers, including zinc dust, may be included in the adhesive composition.
U.S. Pat. No. 3,941,609 (Stern) describes the use of zinc powder, having a controlled size distribution of particles, in paints to provide corrosion protective paints.
Thermoplastic coatings have certain advantages over thermoset resins. They can be less brittle, and have all the advantages of facile application techniques and do not require a separate curing step. On the other hand, thermoplastics have a high coefficient of expansion compared with many thermosets and thus, have had limited utility. One thermoplastic coating is taught in U.K Patent Specification No. 88/20807, published March 1990, which is a priority document for U.S. Pat. No. 5,091,260. The thermoplastic, useful as a sole coating or as a primer coating, is based on mixtures of polyethylene or polypropylene grafted with maleic acid or anhydride containing zinc powder. These thermoplastic coatings may be applied by normal thermoplastic means. These polyolefins grafted with maleic anhydride requires a separate expensive grafting step.
There remains a need for a thermoplastic polymer composition, which functions well as a metal coating and/or metal primer coating, which possess adhesive qualities comparable to thermoset epoxy resins, and is easy to produce and apply to the metal as corrosion protection.